index.js

#!/usr/bin/env node

const characters = ' -=oO0@';
const emptyLine = ' '.repeat(16 * 2 + 1);

code = process.argv[2] || 'sin(t-sqrt((x-7.5)**2+(y-6)**2))';

console.clear();
let output = '';
const callback = new Function('t', 'i', 'x', 'y', `
  with (Math) {
    return ${code.replace(/\\/g, ';')};
  }
`);

let startTime;

function render() {
  output = '\n\x1b[40m  ' + emptyLine;
  output += '\n  ';
  let time = 0;

  if (startTime) {
    time = (new Date() - startTime) / 1000;
  } else {
    startTime = new Date();
  }

  let index = 0;
  for (let y = 0; y < 16; y++) {
    for (let x = 0; x < 16; x++) {
      let value = Number(callback(time, index, x, y));
      let color = '\x1b[37m';

      if (value < 0) {
        value = -value;
        color = '\x1b[31m';
      }

      if (value > 1) {
        value = 1;
      }


      const character = characters[
        Math.floor(value * characters.length)
      ] || ' ';

      output += color + character + ' ';
      index++;
    }

    output += ' \n  ';
  }

  output += emptyLine;
  output += '\x1b[0m\n';
  output += '\n  (t,i,x,y)=>';
  output += '\n  ' + code;
  output += '\n';
  output += '\n';
  process.stdout.cursorTo(0, 0);
  process.stdout.write(output);

  setTimeout(render, 1);
}

render();